1. Field
The present invention relates to a shock absorber.
2. Description of the Related Art
A hydraulic shock absorber for motorcycles includes a cylinder provided in a case in which oil is sealed, a piston provided so as to be slidable in the cylinder, a rod connected to the piston so as to extend outside the cylinder, and a damping force generating unit that controls the flow of oil occurring when the piston slides in the cylinder, to generate damping force. Such a hydraulic shock absorber is provided between a vehicle body and a wheel of a motorcycle in a state in which the case is connected to the wheel side or the vehicle body side and the rod is connected to the vehicle body side or the wheel side.
JP-2011-012806-A discloses a configuration of a damping force generating unit which includes a compression-side valve provided in a compression-side passage through which oil flows when a piston moves to a compression side in a cylinder and an extension-side valve provided in an extension-side passage through which oil flows when the piston moves to an extension side in the cylinder. Here, the damping force generating unit communicates with a subtank that stores oil, on a downstream side of the compression-side valve and the extension-side valve.
In a hydraulic shock absorber having such a configuration, the piston slides in the cylinder of the case when the wheels move up and down in relation to the vehicle body during traveling of a motorcycle. When the piston moves to the compression side in the cylinder, oil compressed by the piston flows into the compression-side passage and the passage is narrowed by the compression-side valve of the damping force generating unit whereby damping force is generated. Moreover, when the piston moves to the extension side in the cylinder, oil flows into the extension-side passage and the passage is narrowed by the extension-side valve of the damping force generating unit whereby damping force is generated.
The compression-side valve and the extension-side valve of the damping force generating unit are formed by a piston body having a port into which oil flows from the compression-side passage or the extension-side passage, and a thin sheet-shaped valve provided on one surface side of the piston body to block the port. When oil flows from the compression-side passage or the extension-side passage into the port, the valve elastically deforms according to the pressure of the oil in a direction of being separated from the surface of the piston body and the oil passes through a gap between the valve and the piston body. The passage of the oil is narrowed by the gap between the valve and the piston body, and the damping force is generated as a result.
For example, in a motorcycle for competition such as motocross, when the motorcycle hits on the ground from a jumping state, the wheels are displaced at a high speed of 10 m/s, for example, in relation to the vehicle body. When a displacement at such a high speed is input to the shock absorber, the piston moves at a high speed in the cylinder. As a result, oil flows at a high flow speed in the cylinder, and a large passage resistance is generated in a portion in which damping force is not generated at a low flow speed (for example, a narrowed intermediate portion of the passage through which oil flows toward the gap between the valve and the valve body). As a result, damping force is generated in a portion other than the gap between the valve and the piston body in which damping force is to be generated, and the shock absorber cannot exhibit its original damping characteristics.